Web-based service providers such as Shutterfly, Inc. provide share sites for users to share their photos, videos, and remarks. To register, a user is only required to provide an email address and a password. The user is then allowed to set up share sites. The owner of a share site can send emails to invite people to become members of the share site, or announce publication of new content at the share site. These features are usually free of charge.
Unfortunately, the share sites have been used increasingly as a platform by spammer to send spam emails. A spammer signs up at the service provider and sets up a share site, and then uses the email service of the share site to send spam messages. The spammer usually does not upload or publish content at the share site because it is not required.
The spam emails have caused significant negative impact on the services of the share sites and the associated web-based service providers. The email spamming from share sites have alienated users to such a degree that some users have opted out of email communications from the share-site service providers such as Shutterfly, Inc. The handling of customer complaints about email spamming and the removals of spammers' account waste much resources of the service provider's customer service.
Email providers such as Google and Yahoo often use semantic-based spam filters to remove spam emails received in users' email accounts. Although this type of anti-spam techniques may be suitable for spam emails received over the Internet, they often cannot provide the most accurate spam prevention for share sites. There is a need for suppressing spam emails at share site with high detection accuracy and low rate of false alarms.